Adiabatic processes

Adiabatic processes happen without external heating or cooling. In Hyperbaric Medicine, the Joule-Thomson effect and adiabatic compression are of interest.

Joule-Thomson effect

'When letting a gas expand adiabatically (= without external heating), the gas will cool down.'

The Joule-Thomson effect was first described by James Prescott Joule (1818-1889) and Sir William Thomson (1824-1907). During adiabatic decompression, most gases at atmospheric pressure behave like this, the only gas which warms upon expansion under standard conditions being hydrogen.

Adiabatic compression

'When compressing a gas adiabatically (ie without external cooling), the gas will warm up.'

Adiabatic compression describes the opposite effect.

Practical relevance: During compression, the gas inside a hyperbaric chamber warms up. The faster the compression, the more the compressed gas will warm up. Compression of a hyperbaric chamber for treatment of DCS to 280kPa "as fast as possible" (e.g., according to US Navy treatment table 6) may lead to a temperature of 40°C or more. Rapid decompression has the opposite effect.

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